A test for microbiome-mediated rescue via host phenotypic plasticity in <i>Daphnia</i>

Phenotypic plasticity is a primary mechanism by which organismal phenotypes shift in response to the environment. Host-associated microbiomes often change considerably in response to environmental variation, and these shifts could facilitate host phenotypic plasticity, adaptation, or rescue populations from extinction. However, it is unclear whether changes in microbiome composition contribute to host phenotypic plasticity, limiting our knowledge of the underlying mechanisms of plasticity and, ultimately, the fate of populations inhabiting changing environments. In this study, we examined the phenotypic responses and microbiome composition of 20 genetically distinct <i>Daphnia magna</i> genotypes exposed to non-toxic and toxic diets containing <i>Microcystis</i>, a cosmopolitan cyanobacterium and common stressor for <i>Daphnia. Daphnia</i> exhibited significant plasticity in survival, reproduction and population growth rates upon exposure to <i>Microcystis</i>. However, the effects of <i>Microcystis</i> exposure on the <i>Daphnia</i> microbiome were limited, with the primary effect being differences in abundance observed across five bacterial families. Moreover, there was no significant correlation between the magnitude of microbiome shifts and host phenotypic plasticity. Our results suggest that microbiome composition played a negligible role in driving host phenotypic plasticity or microbiome-mediated rescue.